Diffusive versus non-diffusive paths to interstellar hydrogen peroxide

Context. Radical chemical reactions on cosmic dust grains play a crucial role in forming various chemical species. Among different radicals, the hydroxyl (OH) is one of the most important, with a rather specific chemistry. Aims. The goal of this work is to simulate the recombination dynamics of hydroxyl radicals and the subsequent formation of hydrogen peroxide (H2O2). Methods. We employed neural-network potentials trained on ONIOM(QM/QM) data, combining multi-reference (CASPT2) and density functional theory calculations. This approach allowed us to model the recombination of hydroxyl radicals on ice surfaces with high computational efficiency and accuracy. Results. Our simulations reveal that the initial position of the radicals plays a decisive role in determining recombination probability. We found that the formation of a hydrogen bond between radicals competes with the formation of hydrogen peroxide, reducing the recombination efficiency, which is contrary to what was expected. This competition reduces the recombination probability for radicals that are initially formed approximately 3 Å apart. Recombination probabilities also depend on the kinetic energy of the added radicals, with values around 0.33 for thermal radicals and a wide range of values between 0.33 and 1.00 for suprathermal OH radicals. Conclusions. Based on our calculations, we provide recommendations for introducing OH radical recombination into kinetic astrochemical models, differentiating between thermal and suprathermal radicals. The recombination behaviour varies significantly between these two cases: while thermal radicals are sometimes trapped in hydrogen-bonded minima, the case of suprathermal radicals varies with the added energy. Our most important conclusion is that OH radical recombination probability cannot be assumed to be 1.0 for a wide variety of cases.